Tissue paper-based materials find extensive use in modern society. Toilet paper, paper towels such as hand towels or household (kitchen) towels, facial tissues and tissue handkerchiefs are staple items of commerce. These products are typically made from papermaking pulp including fibers of both hardwood and softwood types.
Among the most important physical properties of these products are their strength, their softness, their absorbency, primarily for aqueous systems, and their lint and dust resistance. These physical properties are generally tuned for addressing common consumer demand.
Tissue paper products are often exposed to extremely varied strength requirements in the wet and the dry state, respectively. For instance, it must be ensured, in the case of household paper (paper towels) that they retain their strength at least for a specific period of time when exposed to aqueous liquids or moisture-containing food. On the other hand, toilet paper should dissolve in water, sometime after use, in order to prevent the sewage systems from clogging up. Furthermore, toilet paper must not immediately lose its strength properties during use for apparent reasons.
At the same time, tactile properties such as softness are extremely important as tissue paper-based products are intended to come in intimate contact with the body and skin. Therefore, tissue paper-based products must exhibit sufficient softness in order to ensure consumer's comfort.
However, strength and softness are generally conflicting properties. If strength rises, the softness of the tissue paper will decline due to the increase of fiber to fiber bonds. Conversely, if softness increases, the strength is decayed due to limited fiber to fiber bonds.
The prior art describes many processes for achieving a good balance between strength and softness, or increasing one property without detrimentally affecting another.
EP 0 029 269 A1 discloses a multi-layered tissue paper, and tissue paper-based products made thereof, such as toilet tissue and facial tissues, having a smooth and soft top surface. The tissue paper comprises a velutinous top layer being the outwardly facing surface formed from at least 60% by weight of short hardwood fibers, such as Northern Hardwood Sulfite and/or Eucalyptus Hardwood, united to a furnish comprising long softwood fibers. The short papermaking fibers disposed on the outer layer exhibit sufficient free end portions to achieve softness, whereas the long-fiber furnish ensures strength. However, for some applications this tissue paper does not exhibit sufficient strength, primarily in the dry state.
Another common measure for modifying the strength and softness properties of tissue papers consists in adding strengthening and/or softening compositions to tissue-based materials. In this regard, the prior art describes strengthening resins, such as polyamidoamine-epichlorine resins. However, the isolated use of strengthening resins generally provides a tissue paper which is fairly stiff and has almost the haptic properties of normal paper. Consequently, strengthening resins are often used in combination with softening compositions which, in turn, reduce strength since softeners also interact with inter-fiber hydrogen bonds.
WO 94/10381 A1 discloses soft and strong tissue paper webs which can be used in toweling, napkin, facial tissue, and toilet tissue products. The tissue paper webs generally comprise a cellulose-based furnish, such as a mixture of Northern softwood kraft and eucalyptus fibers, and a chemical softening composition comprising a cationic surfactant as softener. However, surfactants weaken the bonds between fibers in the tissue webs. As a result, the tissue paper webs do not exhibit sufficient strength. The loss of strength due to the surfactant is hence compensated by the addition of a binding resin, such as a polyamidoamine-epichlorohydrin resin.
Likewise, U.S. Pat. Nos. 5,397,435 and 5,312,522 disclose tissue paper-based products such as paper towels, facial tissues and toilet tissues, comprising a combination of a chemical softening composition containing a surfactant such as a quaternary ammonium compound, and a strengthening resin such as a polyamide-epichlorohydrin resin. However, as outlined above, the use of softening and strengthening chemical compositions in combination generally does not provide a significant improvement, because the effect of one composition (e.g. softening or strengthening) is often compromised by the opposite effect caused by the other.
In addition, it can be desirable to reduce the amount of chemicals such as softening and/or strengthening chemical compositions in tissue paper. This applies primarily to cases where these chemical compositions tend to irritate the skin or trigger allergic reactions in some users. Furthermore, the biological degradability of some softening and/or strengthening chemical compositions in the environment has raised concerns.
WO 96/06223 A1 proposes a tissue sheet composed of northern softwood kraft and eucalyptus hardwood kraft, comprising a combination of “debonding” agents and strengthening agents added in a layer-wise fashion, in order to maximize the effectiveness of each additive while minimizing the interaction of the additives with each other. However, the addition of different chemical compositions in a layer-wise fashion significantly increases the complexity of the manufacturing process.
These are typical examples for the generally occurring problem in the art of tissue paper making that the above-mentioned properties conflict with each other in so far as attempts to improve one property is detrimental for another.
Further, it has been noted that, in tissue paper making processes, the primary (embryonic) fibrous webs containing eucalyptus pulp fibers sometimes do not show the desired strong adhesion to the Yankee cylinder when the web is subjected to the final drying and creping steps.
Starting from commonly used tissue paper products using mixtures of softwood and eucalyptus pulps, it is desired to provide tissue paper webs and products having improved properties, in particular improved strength and good softness.
It is also desired to provide a process for manufacturing such tissue paper webs. According to one aspect, a process is provided which involves an improved adhesion of the “primary” fibrous webs to the Yankee cylinder during the final drying and creping steps of the process.